1. Field of the Invention
The present invention relates to image transfer apparatuses, image fixing apparatuses, and registration apparatuses in which a transfer material is transported into a nipping portion between a first rotating body rotated by a rotating force provided by a drive source and a second rotating body that is pressed against the first rotating body with a predetermined pressing force.
2. Description of the Related Art
In an electrophotographic image forming apparatus, a photosensitive material in the form of a drum or a belt is charged, and an electrostatic latent image is formed on the photosensitive drum or belt while it is rotated. The latent image is then visualized by causing toner to attach to it using a developing apparatus, whereby a toner image is formed on the image carrier, i.e., the drum or belt. The toner image is transferred onto the recording medium that is transported, such as a sheet of paper or an OHP film, either directly or via an intermediate transfer material that may be in the form of a belt.
In such an image forming apparatus, the transfer of the toner image on the image carrier onto the recording medium or the intermediate transfer material is carried out by an image transfer apparatus.
FIGS. 14A-14C show an image transfer apparatus in a conventional image forming apparatus, such as disclosed in Japanese Patent No. 2883916 or Japanese Laid-Open Patent Applications 61-90167.
With reference to FIG. 14A, the image transfer apparatus includes a displacement roller 201 disposed upstream of an image transfer position along a sheet transport path where an image is transferred by nipping a sheet. The displacement roller 201 is displaced downward as a sheet of transfer material 204 passes thereon, as shown in FIG. 14B, whereby the pressing force of a pressure roller 203, which is coupled to the displacement roller 201 via a connecting arm 202, is adjusted depending on the amount of movement of the displacement roller 201. In this way, the impact of the transfer material 204 as it enters the sheet-nipping image transfer position is reduced, as shown in FIG. 14C.
Japanese Laid-Open Patent Application No. 6-274051 discloses a mechanism in which a gap is produced at a nipping portion by moving a pressure roller with a drive force provided by a drive unit. Specifically, an arm that supports the pressure roller is pressed down by the rotating force of an elliptic cam in order to produce a gap at the nipping portion in advance. The gap is subsequently eliminated and the pressure roller is pressed onto a drum upon entry of a sheet into the nipping portion between the roller and the drum.
Japanese Laid-Open Patent Application No. 2006-317627 discloses that, in order to reduce a load torque variation that occurs in a secondary transfer unit in an image forming apparatus due to the entry or exit of a thick sheet, the inertia of a pressure roller in the secondary transfer unit is minimized.
In the aforementioned technologies disclosed in Japanese Patent No. 2883916 and Japanese Laid-Open Patent Applications 61-90167, the presence of the displacement roller and the connecting arm increases the size of the transfer apparatus. In addition, because the transfer pressure varies depending on the thickness of the sheet, it is difficult to obtain appropriate transfer/nipping conditions. Since the purpose of nipping a sheet in a transfer apparatus or a fixing apparatus is to give a desired pressure to the toner image, a constant pressure needs to be imparted regardless of the thickness of the sheet. However, in the apparatuses according to these publications, nipping conditions vary depending on the depressing force of the connecting arm, which varies depending on the thickness of the sheet. As a result, a defective transfer or fixing may easily occur when a thick sheet is used.
In the aforementioned technology according to Japanese Laid-Open Patent Application No. 6-274051, in order to allow the arm to be moved at high speed against a pressing force, the actuator including the arm needs to have high rigidity and be able to provide a high torque. In recent years, the sheet transport speeds have been increased for productivity improvement purposes. There is also a demand to increase the image area on a sheet (i.e., reduction of the margin, or “borderless” image). Thus, in accordance with this technology, it is necessary to perform an operation of switching from a spaced-apart condition to a nipped condition in the nipping portion instantaneously. For example, in an image forming apparatus in which a sheet is transported at the speed of 200 mm/s, if it is desired to transfer an image onto the sheet 2 mm from its front edge, it is necessary to switch from a spaced-apart state to a nipped state approximately 0.01 second after the entry of the sheet into the nipping portion. However, it is difficult to realize a drive control mechanism that produces a torque with which the switching can be performed at such high speed. Furthermore, the impact upon pressing one roller onto another tends to produce vibrations.
In the aforementioned technology according to Japanese Laid-Open Patent Application No. 2006-317627, the pressing roller is pressed against an opposite roller via an elastic member. The pressing roller is required to have sufficient rigidity so that a uniform pressure can be imparted along the axis of the roller. Thus, there is a limit to which the inertia of the roller can be reduced.
In a conventional apparatus utilizing a pressing force, such as the aforementioned transfer apparatuses, in an electrophotographic apparatus as an image forming apparatus, a transfer material, such as a sheet of paper, may be transported to a nipping portion between a fixed roller and a pressing roller that is pressed against the fixed roller in order to transfer an image formed on the fixed roller onto the transfer material. Alternatively, in such a transfer apparatus, a transfer material may be passed through a nipping portion between a belt transported on a roller and another roller, in order to transfer an image on the belt onto the transfer material. In such transfer apparatuses, there is the problem that a load torque variation occurs when the transfer material enters or exits the nipping portion, resulting in a speed variation in the transfer material.
In the aforementioned conventional art according to Japanese Laid-Open Patent Application No. 6-274051, in which the roller and the drum are spaced apart in advance, a mechanism is required to produce a gap greater than the thickness of the transfer material. Such a separating mechanism requires a drive apparatus capable of producing a large torque.